Howell’s triteleia (Triteleia howellii) COSEWIC assessment and status report: chapter 2

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Executive Summary

Species information

Howell’s triteleia, Triteleia howellii (S. Wats.) Greene, is a member of a genus of 14 species in the Liliaceae in North America. Three species occur in British Columbia and Canada. Triteleia howellii is a perennial herb from a deep, straw-coloured, fibrous-scaly, nearly globe-shaped, bulb-like corm. The erect, flowering stem is 20-50 cm tall with one or two smooth, slender, linear basal leaves. The leaves are 20-40 cm long, 3-8 mm wide, sheathed at the base and have entire margins. The flowers consist of six whitish to blue, vase-shaped to narrowly bell-shaped, fused segments forming a 1.5-2 cm long tube. The corolla lobes, which are about as long as the tube, are in two, spreading, petal-like whorls. The outer three are broadly lanceolate, the inner three are oblong-egg-shaped and all are slightly ruffled. The fruit consists of a stalked, egg-shaped capsule containing black, rounded seeds.

Distribution

Triteleia howellii ranges from southwestern British Columbia, south through Washington and Oregon to northern California. In Canada, T. howellii is only known from southeastern Vancouver Island in southwestern British Columbia.

Habitat

Triteleia howelliisites are restricted to southeastern Vancouver Island in British Columbia in the Quercus garryana (Garry oak) ecosystem, which is within the Coastal Pseudotsuga menziesii (Douglas-fir) Zone of southeastern Vancouver Island, islands in the Gulf of Georgia, and a narrow strip of the adjacent mainland. This zone is in a rainshadow belt created by the Olympic and Vancouver Island mountains, resulting in a Mediterranean climate with warm, dry summers and mild, wet winters. More specifically, T. howellii occurs in Quercus garryana woodlands and in highly disturbed sites dominated by weeds in private yards and on roadsides. The Quercus garryana woodland in the Cowichan Garry Oaks Preserve is classified as a Quercusgarryana/Dactylis glomerata (orchard grass) plant community and is characterized by deep, dark soils up to a metre in depth. Triteleia howelliialso occurs in a Quercus garryana-Arbutus menziesii (Arbutus) stand at the base of rock outcrops, where the shrub layer is more prominent with high cover of Mahonia aquifolium (tall Oregon-grape) and Holodiscus discolor (oceanspray).

Biology

There is little known about Triteleia howellii in terms of biology throughout its range. Reproduction is through division of the corm, by the production of numerous cormlets, and by seed.

Population sizes and trends

Triteleia howellii has been collected at 12 sites in Canada, all of which are located on southeastern Vancouver Island. Nine of the twelve sites have been confirmed since 1997, while the status of the plants at the remaining three sites is unknown and the populations are likely extirpated. Population areas range from small (1 m²) to over three or four hectares, while plant numbers range from a single plant to over 450 plants.

Limiting factors and threats

The most direct and immediate threat to Triteleia howellii is habitat destruction.  The Quercus garryana communities that are limited to the southeastern side of Vancouver Island and some of the Gulf Islands have been heavily urbanized. The suppression of fire and the spread of introductions has also been a limiting factor. One of the most devastating introduced species is Cytisus scoparius (Scotch broom), which has become a dominant shrub on xeric, exposed sites throughout much of southeastern Vancouver Island and the Gulf Islands. Furthermore, dispersal into new sites is likely limited and some of the populations of T. howellii contain very few plants and could be at risk of inbreeding depression, genetic drift and loss of fitness.

Special significance of the species

Triteleia howellii occurs in a unique ecosystem in Canada, the Garry oak woodland, which itself occurs within a limited habitat type, the Coastal Douglas-fir zone of southeastern Vancouver Island, several islands in the Gulf of Georgia and a narrow strip of adjacent mainland in British Columbia. The Garry oak ecosystem is a unique habitat for Canada and is at the northern limit of the vegetation type that occurs more commonly to the south. The northern limit of the species also occurs in this region. The importance of these peripheral populations, especially with respect to their genetic characteristics, has yet to be studied adequately. The bulb-like corms of this species are edible and like other related species may have been used by native peoples as a food source.

Existing protection or other status designations

Globally T. howellii is ranked "G3G4", indicating, it is "apparently secure." Triteleia howellii is considered "imperiled" in California.  Nationally, T. howellii is “N2” and provincially, it is considered by the BC Conservation Data Centre of the BC Ministry of Sustainable Resource Management to be “red-listed” and “S2” or imperiled, the second most critical rank. However, there is currently no specific endangered species legislation in place for the protection of vascular plants in British Columbia that have been given this critical rank.

Summary of status report

Triteleia howellii is known from only nine extant populations in British Columbia, some of which have very few individuals, putting them at risk of inbreeding depression, genetic drift and loss of fitness. Aggressive exotic species and the degradation of the ecosystems that T. howellii occur in, as well as direct habitat destruction, threaten its long-term persistence in British Columbia; consequently, dispersal into new sites is very limited. There is currently no specific legislation in place for the protection of rare and endangered vascular plants in British Columbia. The populations of T. howellii in British Columbia are at the northern extent of their range and may represent populations that are genetically distinct and important for the long-term survival and evolution of the species.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) determines the national status of wild species, subspecies, varieties, and nationally significant populations that are considered to be at risk in Canada. Designations are made on all native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fish, lepidopterans, molluscs, vascular plants, lichens, and mosses.

COSEWIC Membership

COSEWIC comprises representatives from each provincial and territorial government wildlife agency, four federal agencies (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biosystematic Partnership), three nonjurisdictional members and the co-chairs of the species specialist groups. The committee meets to consider status reports on candidate species.

Definitions

Species

Any indigenous species, subspecies, variety, or geographically defined population of wild fauna and flora.

Extinct (X)

A species that no longer exists.

Extirpated (XT)

A species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A species facing imminent extirpation or extinction.

Threatened (T)

A species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)^Footnotea

A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events.

Not at Risk (NAR)^Footnoteb

A species that has been evaluated and found to be not at risk.

Data Deficient (DD)^Footnotec

A species for which there is insufficient scientific information to support status designation.

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list.